Method for reduction of aluminum with improved reduction cell and anodes

ABSTRACT

A system for the fused floride electrolysis of aluminum in a potcell wherein a carbonaceous anode and potlining are preserved against deterioration by impregnating their respective surfaces with a pitch-fluoride mixture and a pitch-graphite mixture. Atmospheric oxidation of anode surfaces is prevented so maximum anode cross sectional area is preserved and, hence, has minimum resistance for current travel through the anode and through the underlying electrolyte. The more dense carbon surface also reduces corner erosion of anodes. Impregnation is preferably carried out with one or more applications of aluminum fluoride suspended in a high melting, low volatile coal tar pitch. An improved potlining protects against electrolyte penetration and circulating metal erosion of the potlining surface, and improves conductivity; the lining bottom is sealed and smoothed with graphite suspended in molten pitch, which is absorbed by capillary attraction, and which increases conductivity. The lining walls are sealed and smoothed with a similar mixture of fluorspar and pitch, which decreases conductivity.

,[22] Filed:

llnited States Patent 1191 Johnson [54] METHOD FOR REDUCTION or ALUMINUMwrrn IMPROVED REDUCTION CELL AND ANODES [76] Inventor: Arthur F.Johnson, 203 Creole Ln.,

North Gate Urban Farms, Franklin Lakes, NJ, 07417 Sept. 13,-1972 [21]Appl. No.: 288,706

Related US. Application Data [62] Division of Ser. No. 172,047, Aug. 16,1971.

[52] US. Cl. 204/67, 204/290 R, 204/294 [51] Int. Cl C22d 3/12, BOlk3/04, B0lk3/08 [58] Field of Search 204/67, 294, 290 R [56] ReferencesCited UNITED STATES PATENTS 3,303,119 2/1967 Dell 204/294 3,236,7532/1966 Skantze et al. 204/67 3,442,786 5/1969 Clukey et al 204/294 X3,716,471 2/1973 Primary ExaminerJohn H. Mack Assistant Examiner-D. R.Valentine Attorney, Agent, or Firm-JamesJ. Burke Cullenetal. 204/67 x1451 Jan. 22, 1974 [57] ABSTRACT A system for the fused florideelectrolysis of aluminum in a potcell wherein a carbonaceous anode andpotlining are preserved against deterioration by impregnating theirrespective surfaces with a pitch-fluoride mixture anda pitch-graphitemixture. Atmospheric oxidation of anode surfaces is prevented so maximumanode cross sectional area is preserved and, hence, has minimumresistance for current travel through the anode and through theunderlying electrolyte. The more dense carbon surface also reducescorner erosion of anodes. impregnation is preferably carried out withone or more applications of aluminum fluoride suspended in a highmelting, low volatile coal tar pitch. An improved potlining protectsagainst electrolyte penetration and circulating metal erosion of thepotlining surface, and improves conductivity; the lining bottom issealed and smoothed with graphite sus pended in molten pitch, which isabsorbed by capillary attraction, and which increases conductivity. Thelining walls are sealed and-smoothed with a similar mixture of fluorsparand pitch, which decreases conductivity.

4 Claims, 3 Drawing Figures METHOD FOR REDUCTION OF ALUMINUM WITHIMPROVED REDUCTION CELL ANODES This is a division of U.S. ApplicationSer. No. 172,047, filed Aug. 16, 1971.

BACKGROUND or THE INVENTION 1. Field of the Invention The presentinvention relates generally to aluminum electrolysis in a moltencryolite bath, commonly referred to as the Hall process. Moreparticularly, the present invention relates to avoiding atmosphericoxidation of anodes, aproblem which has long plagued the industry.Additionally, the present invention relates to means for increasing celllife.

In theproduction of aluminum by the Hall process, large numbers ofindividual electrolytic cells are employed. Each cell has a carbonaceouslining forming the cathode, with cathode collector bars buried therein.Suspended above each cell on iron rods are a plurality of carbon anodes.As electrolysis proceeds carbon on the anodes is gradually consumed andthey are lowered further into the bath. Because of the high temperaturesinvolved, carbon on the sides and top of the anodes exposed to theatmosphere tends to oxidize (i.e., to burn in air), considerablyreducing the anode size. Anodes are further eroded in contact with thebath, particularly 'at the comers, which are current concentrationpoints. The molten bath tends to erode the lining and to penetrate intocracks and pores therein, eventually causing the latter to becomedimensionally unstable. At that point, a shut-down for re-lining isrequired.

2. Prior Art In the past, many attempts have been made to improve theresistance to decomposition of both anodes Historically, the greatestsavings in power consumption in aluminum reduction potcells has comeabout by using lower and lower anode current densities with larger andlarger anode areas, providing thereby greater areas in the underlyingelectrolyte and cathode for the current to flow at lowerelectricalresistance. Atmospheric oxidation of anode periphery not submerged inthe electrolyte reduces anode area, increases carbon consumption, and sois doubly undesirable.

One of the oldest methods of preventing surface oxidation of the anodes,since the commercialization of the Hall process about 80 years ago, hasbeen to splatter molten bath on the upper parts of the anodes or dipthem in the molten electrolyte bath. Stirring tools or tools especiallymade for the purpose have long been conventionally used to splatter andcoat the anodes with a fraction of inch of bath after they become redhot during operation. Also for years or more, cryolite powder has beendusted on red hot anodes to which it adheres and wets. However, aslaborbecomes more expensive this type of hot hand work around a potcellbecomes prohibitively expensive.

In the anodes are dipped in a moltenbath in a rodding room productionline operation prior to being introduced into the aluminum reductionfumance, they must be heated to red heat to make the bath adhere,

and it never adheres well enough so that some is not broken off inhandling and transportation, and it can be contaminated with unwantedsilica and other impurities on the plant floor swept to recover the bathparticles.

For 20 years or more it has been common practice to use as much as a 6inch layer of alumina on the bath crust of a potcell to prevent theatmosphere from contacting the anodes where their temperature is highestnear the bath. Since about t 1.9 pounds of alumina are used per pound ofaluminum produced, it has been advantageous to preheat alumina in thisway while at the same time preventing atmospheric air from contactingand oxidizing the anode surfaces which are at the highest temperature.In more recent years more accurate control of the alumina content ofmolten electrolyte and better operating efficiencies .resultingtherefrom have made it expedient to add most of the alumina almostcontinuously, directly to the molten electrolyte. This makes itincreasingly important to prevent surface oxidation of anodes by meansother than thick layers of alumina on the crust of the electrolyte.

Anode oxidation by the atmosphere can be .greatly reduced on acommercial scale by coating prebaked anodes with a layer of aluminumbetween one thirtysecond and 1 inch thick, depending on whether thealuminum is sprayed or cast on the surface. The disadvantage of thismethod is the inevitable oxidation of some of the aluminum when it isbeing applied and, again,

when subjected to a week or more of heat from the red hot anode which itcovers. The cost of the aluminum so oxidized is a substantial part orgreater than the cost of anode carbon which would be lost without suchprotectron.

U.S. Pat. No. 3,303,119 of Dell discloses coating anodes with a thinmetal sheet attached with a bitumin mastic. The U.S. Pat. of Clukey etal. No. 3,442,786, discribes coating anodes with a stream of aluminumdirected against them. The U.S. Pat. of Skantze et al., No. 3,236,753discloses coating anodes with a cryolite mixture by dipping them in amolten bath.

My own U.S.-Pat. Nos. 3,372,105 and No. 3,428,545 disclose the use of agraded refractory material bonded to a flexible strip which iswrappedaround the outer surface of an anode to render it immune tooxidation.

While these measures will indeed prevent unwanted oxidation, they aredisadvantageous to some extent by being either expensive in materials orlabor of application, imperfect in protection or in introducing unwantedelements such as phosphorus, sulphur, titanium, silicon or otherelements harmful to ampere efficiency, life of the cathode potlining, orthe purity of aluminum reduced.

In my prior U.S. Pat. No. 3,457,149, cathode potlining decomposition isprevented by impregnating the potlining with molten halide material ofrelatively low melting point, while applying a vacuum to the potlining.Also, in U.S. Pat. No. 2,270,199, of I. Thrune, there is disclosed meansof applying to a once-baked graphite article a further coating ofgraphite paste made from graphite powder, a binder of liquid coal tar,an excipient and hardening agent, the excipient preferably being avolatile liquid boiling below about C., and an antioxidant. This coatingis baked on. The fin ished article will have a smoother surface and befreer from minute cracks or pores than was the article after a singlebaking.

OBJECTS OF THE INVENTION Yet another object of the present invention isto provide means of decreasing electrical resistance between anode andanode rod and collector bar and cathode in aluminum reduction cells.

Still another object of the present invention is to provide a method ofimpregnating aluminum reduction cell anodes and cathodes.

Various other objects and advantages of the invention will become clearfrom the following description of embodiments thereof, and the novelfeatures will be particularly pointed outin connection with the appendedclaims.

THE DRAWINGS Reference will hereinafter be made to the accompanyingdrawings, wherein:

FIG. I is a vertical cross section through an aluminum reduction cellemploying the protective coatings of the present invention;

FIG. 2 is a side elevation of a prebaked carbon cell lining block orsegment being coated in accordance with the invention; and

FIG. 3 is an end elevation of FIG. 2.

SUMMARY OF THE INVENTION In one aspect, the present invention comprisesimpregnating the top and sides of aluminum reduction cell anodes withaluminum fluoride or other compounds normally required in aluminumreduction baths and which are depleted by volatilization and gradualabsorption into the cell lining. This increases cell operatingefficiency by preventing atmospheric anode oxidation and, at the sametime, maintains bath composition at a constant level with respect to thecompounds added. The carbonized pitch presents a more dense carbonsurface to the bath, even after melting of the fluoride or otheringredient, lessening comer erosion and other undesirable effects. Theprotected anode provides maximum anode area and lower electricalresistance, not only through the anodes but also within the bath betweenanode and cathode. This allows higher currents to be used with resultantproduction increase.

A further aspect of the present invention comprises impregnating thecathode bottom with a penetrating graphite coating which seals up minutecracks and pores therein and, by preventing penetration by the moltenbath or molten aluminum, significantly increases cell life. The graphitealso increases conductivwith a fluorspar-pitch mixture which decreasesconducity. Similarly, the cathode side walls are-impregnated tivitywhile at the same time sealing cracks, etc.

Another aspect of the invention is in applying the above-describedmaterials so they penetrate into and become integral with the underlyingcarbon.

DESCRIPTION OF EMBODIMENTS Generally, these coatings consist of a solidbitumastic adhesive such as coal tar pitch and an electrolytic bathingredient like aluminum fluoride, cryolite, sodium fluoride, sodiumcarbonate, calcium fluoride or alumina, mixed with the bitumin. Suchadhesive coating mixtures are applied to the hot baked anode surface orhot surface of the anode sides and top, to densify and protect thesurface.

When the baked cathode potlining is treated, graphite is mixed with thecoal tar pitch and applied to a heated surface of the potlining, whichabsorbs the bitumin and graphite into the surface pores and cracks bycapillary attraction, and thereby densities and lessens penetration ofthe fused bath into the potlining during potcell operation.

The method of applying and sealing the coatings comprises preheating andevacuating relatively small surface areas of the carbonaceous anode orcathode immediately before applying the adhesive coating mixture. Only alimited amount of the mixture, that which can be absorbed in thesurface, is applied at one time. After application the surface isfurther heated, gradually, to burn off most of the volatiles in thepitch and cause the mixture to penetrate further into the surface beingtreated. As that surface area cools, the pore spaces from which air hasbeen at least partly expelled by heating or other evacuating means drawinward the coating mixture to fill the vacuum. When the heating flame orradiant mass moves over the surface to be sealed, followed closely by anapplicator of the coating mixture, a continuous and economical sealingprocess results. The coal tar pitch may be kept molten and continuouslystirred with powdered fluoride or other ingredient of the bath orgraphite, or the premixed and proportioned coating may be cast in blockfonn which is pressed against the preheated carbon surface to be sealedand thereby melted where and as needed.

Many other binders may be used as adhesives, such as other hydrocarbons,or carbohydrates like molasses, but coal tar pitch of high melting pointand low volatile content makes more dense carbon and a more adherentcoating.

The invention may be carried out by heating and evacuating a part of theanode surface quickly, with radiant or conducted heat or a clean gasflame, to a temperature of about 300 C., and applying proximate theretoa melted coal tar pitch, also at 300 C., in which there is suspended 5to percent of an electrolyte ingredient such as aluminum fluoride ormixture of these. As the anode heater is moved along the anode surface,the pitch-A11 mixture is applied in only sufficient quantity to beabsorbed into the surface. A second heating step gradually heats theimpregnated surface to a temperature sufficient to drive off most of thevolatile content of the pitch, which is preferably very low. A preferredmixture for this operation is three parts of a high-melting,low-volatile coal tar pitch with one part of dry aluminum fluoride (-300mesh) suspended therein. The above-described fired coating essentiallyfire proofs the anode under conditions and for the duration of potcelluse. However, a second treatment may be desireable, with the mixture inthis instance containing either aluminum fluoride or other molten bathingredients. For example the second coating may comprise sodium fluorideor a mixture of aluminum fluoride, sodium fluoride, sodium carbonate,cryolite and alumina. If the second coating is applied directly afterfiring of the first coating, preheating the surface is not required.When this procedure is followed, the pitchadditive mixture is forcedinto the interstices of the anode carbon surface, tightly sealing these.When all the anode sides and top which would be exposed to atmosphericoxidation are sealed, the anode is ready to be used in a potcell forreduction of aluminum. As the anode heats during operation, its upperportions gradually heat and carbonize the absorbed pitch, which shouldhave a high melting point and coking value. If the heating of the anodeis gradual in the potcell, any remaining pitch volatiles willtend toescape inwardly from the anode areas away from the sealed surface. An

I important feature of this practice of the invention is the fact thatthe anode surface when sealed with pitch is more dense and difficult tooxidize even when the amount of aluminum fluoride or cryolite admixedwith the pitch is relatively low. It appears at least possible that areactive mechanism is present (aluminum fluoride is known as aFriedel-Crafts reagent).

An an alternate to using a moltenmixture of fluoride and coal tar pitch,a precast slab or block of the mixture may be rubbed on the anodesurface which may then be somewhat hotter than 300 C. to melt but notvolatilize the pitch ingredients.

When the above temperatures are used for preheating and application ofthe mixture, a coal tar pitch of the following specifications givessatifactory results. The temperature of application is chosen so that novolatiles are given off by the pitch during initial application.

EXAMPLES OF COAL TAR PITCH SPECIFICATIONS bottom end, which is immersedinto the molten bath at about 960 C. Since only the'upper sides and topof the anode are not immersed in the bath, it is only three upperportions which need be coated with the nonoxidizing coating. Actually,all binders containing carbon which carbonizes will have some carbonsurface exposed to oxidation, andat red heat (about 500 C) such carbonwill oxidize leaving the bath ingredient particles exposed to protectthe anode surface from air until it is submerged in the molten bathfusion.

Bath ingredients which are suitable for practice of the invention inpreventing oxidation of anodes by the atmosphere are: aluminum fluoride,sodium fluoride, sodium carbonate, cryolite, calcium fluoride, lithiumfluoride, magnesium fluoride and alumina or mixtures thereof. Alumina isnot preferred, in that it leaves a more porous coating through which theatmosphere more easily penetrates. Where alumina is added to an adhesivebinder such as molasses, cryolite should be pressed into the surface ofthe anode after it has been wetted by the molasses-alumina mixture.Alumina does have the advantage of having a coefficient of expansioncloser to that of carbon than cryolite.

Aluminum fluoride is the material of choice, at least for the firstapplication, because of its exhibited ability to penetrate with thepitch into the surface. Aluminum fluoride mixtures with pitch penetratea carbon surface as much as three times the depth penetrated by cryolitepitch mixtures under similar conditions of application. As burning offof the volatiles from the first application will leave at least someporosity, mixtures of sodium fluoride, sodium carbonate, cryolite,calcium fluoride and alumina, which dont penetrate as readily may beemployed in the second impregnation. Sodium fluoride or sodium carbonatemay be mixed with alumi- Softening Pt. C-A

l l5C. Distillation: 0-270C. Benzene Insoluble 30. 42'7: 0-300 QuinolineInsoluble I 1.51 0-360 Conradson Coking value I 59.39% 0-400 S ecificGravity 25C./25C. L320 Ash 0.167: Analyses:

' carbon hydrogen sulphur Instead of the above described adhesive, coaltar num fluoride on either the first or succeeding applicabinders may beused with or without hardening agents like carbon tetrachloride whichconvert the tar to pitch upon heating. In fact, almost any adhesive maybe used which will bind pulverized bath ingredient particles to acarbonaceous anode without introducing elements harmful to either theelectrolytic bath or the aluminum reduced therefrom. Hydrocarbons orcarbohydrates which carbonize with the least possible loss of volatilematter are best to make a dense, nonporous surface which will notoxidize. Specifically, molasses may be used mixed with pulverized bathingredients like cryolite, alumina or aluminum fluoride, and heatedabove boiling temerature before application to the anode face, whichshould be preheated at least equally hot so the mixture may be appliedas thin as possible. Immediately after the molasses-solid mixture isapplied, additional powdered material may be pressed into the wettionssince mixtures of 40 percent AIR, and 60 percent NaF begin to sintertogether to form cryolite at temperatures as low as 500 C. and providemore perfect oxidation resistance.

The adhesive hinder or mixture with non-oxidizing ingredients should notcontain any substantial amounts of the following, which are eitherinsoluble in or deletemust be added as make-up for these losses, whetherthey are ultimately recovered or not.

It is to be noted that anode makers sometimes provide anodes with slopedshoulders at the top and cut-off or rounded corners, for the simplereason that these are the areas subject to the most severe oxidation anderosion. With the impregnating coatings of the present invention suchcurrent-limiting shapes may be and preferably are avoided. Even when thealuminum fluoride or other bath ingredient melts and flows into thebath, the carbonized pitch surface is more dense than an ordinary anodesurface and erosion due to electrical and other effects is reduced.

As an alternative to the second step noted hereinabove, whereincryolite, alumina or other bathingredient particles are mixed with pitchand impregnated over the aluminum fluoride-pitch, these particles may beblasted directly onto the hot surface of the anodes immediatelyfollowing the first aluminum fluoride-pitch application, beforevolatiles are burned off. These particles will become embedded in thehot, still soft surface and fill what pores there may be.

For the highly penetrating aluminum fluoride-pitch treatments, theparticle size of the fluoride or bath ingredient mixed therewith shouldof course be small: 80 percent minus 300 mesh (tyler screen) ispreferred. With other, less penetrating mixtures, size-gradedparticulates will produce a more dense surface. A mixture of 50 percent28 mesh +48 mesh and 50 percent l mesh +300 mesh is typical.

When the invention is applied to the protection of potlining against theerosion of molten bath due to electromagnetic circulation and due tosodium penetration which decomposes and heaves the potlining, theprocedures outlined above for anode protection are generally followed.In operation, it is desired that the cell bottom be conductive but thatthe sides, while still part of the cathode, be relativelynon-conductive. Thus, for bottom coating, graphite powder is substitutedfor powdered bath materials. To make the side potlining electricallynon-conducting, a calcium fluoride (Fluorspar) -pitch mixture may beused either in a continually molten or pasty condition or premixed andcast in solid block. In any case the preferably prebaked side potliningsurface is heated over a little area at a time and the mixture appliedto the extent that it absorbs into the potlining surface, coats it andmakes it more dense and hard.

When a carbonaceous bottom (cathode) potlining is treated, it haspreferably been already baked. Surface parts are then reheated inrelatively small areas at a time, evacuated, and the pitch-graphitemixture containing to 70 percent graphite is applied in a continuallymixed liquid or pasty from, or in a premixed proportion and then cast inblock form which may be rubbed against the hot, evacuated cathodesurface to seal it. Where a rammed potlining is used (rather than apotlining built of prebaked blocks), and the potlining is baked out bypassing current through the potlining from the anodes which rest on it(or are spaced from the potlining with perhaps an inch of crushed coketo act as a resistance heater), the hot cathode potlining bottom isconveniently sealed by cleaning it and rubbing thereon a precast blockmade of a mixture of graphite powder and pitch. Since the potlining areais very hot and the fumes from the pitch irritating, a handle with pipeattached to hold the block is necessary.

Where large and deep cracks have formed in the bottom potlining,molten-pitch continuously mixed with graphite powder is squirted underpressure into the cracks. After the mixture has time to bake enough sono more fumes are evolved from the pitch, an .additional layer isapplied. Where prebaked carbon blocks are used to build the cathodepotlining, only the top surface need ordinarily be treated with thegraphitepitch mixture before or after the blocks are set in place, butan application of the mixture on the upper surface of the lining betweenblocks is preferable after it has baked, as it helps to seal thesenarrow spaces or cracks that develop in baking them.

An application of the pitch-graphite mixture to the inside of the slotin potlining blocks adapted to receive the collector bar, and the holefor the stub of prebaked anodes, improves electrical contact, providedthe mixture is baked sufficiently to drive off all or most of the pitchfumes before the cast iron is poured in the potlining block to securethe steel cathode collector bar thereto or before the cast iron ispoured in the anode hole to attach the steel anode stub thereto. Withless RI heat from the contact, the anode top is cooler and suffers lessatmospheric oxidation.

The invention may be better understood by reference to the drawings,where FIG. 1 illustrates a potcell cross section which is conventionalexcepting that novel kinds of coatings of the invention have beenapplied to the anodes to protect them from oxidation and to the side andbottom potlining to inhibit erosion and sodium penetration. FIG. 2illustrates a side view and FIG. 3 and end view of a prebaked potliningsegment for use in the bottom potlining of the potcell of FIG. 1, duringcoating with a graphite-pitch mixture.

The apparatus illustrated in FIG. 1 comprises the usual rectangularsteel shell with a flat bottom 1 and upright sides 2, supported andreenforced by structure not shown. The refractory alumina lining 3 linesthe shell around the steel bottom and sides, and the carbonaceouscathode potlining and side potlining segments 4 which are prebaked andmade in the usual manner from anthracite coal, tar and pitch binder.Bottom lining 4 has a graphite-pitch coating 6 in accordance with theinvention, with reference to FIG. 2 and 3. As common in potcells madewith prebaked segments, rectangular steel collector bars 5 arepositioned in each segment and held there by pouring cast iron 14between the carbonaceous segment 4 and steel bar 5. However, before thecast iron is poured, the area of carbon surface which the cast ironcontacts is sealed with a graphite-pitch mixture in a manner similarlydescribed with reference to FIGS. 2 and 3. In a similar manner, thecalcium fluoride (fluorspar)-pitch coating 7 is applied to the sidepotlining segments. The process may be performed on individual segmentsbefore the segments are assembled to form the potlining and repeatedafter the lining is complete.

After the cell has been operating as a reduction cell for a few days, itaccumulates a layer of molten aluminum 8 reduced from the fusion ofelectrolytic bath 9 which overlays it and on which the crust 10 isnormally present and in which a variety of lengths of anode carbons 11are suspended depending on the number of days in which they have been inservice. The extremes of length illustrated are shown by the new anode11A, which is about 18 inches high and the old anode 11B, only 7 incheshigh, which is called an anode butt be- 9 cause it is about readyto betaken out and replaced by a new anode. These anodes have been protectedagainst atmospheric oxidation by one or more impregnating treatments 12applied before introduction into the 'potcell as new anodes, but only onthe anode top and sides down as far as the molten electrolyte. The

bottom of the anode and the sides about 6 inches up from the bottom needno protective coating (the coating may be applied to these areas ifneeded for make-up purposes)".

Current for electrolytic reduction enters the reduction cell through thesteel anode studs 13 suspended by overhead structure not shown, andsecured to the anodes 11 by cast iron poured after the prebaked anodeshave been baked and after the anode stub hole has been coated with agraphite-pitch mixture 6 to make the surface in contact with the castiron more electrically conductive.

A preferred method of applying the novel coatings and one which may beusedin coating either anodes or potlining is illustrated in FIG. 2 andFIG. 3, where the potlining segment top surface is coated with thegraphite-pitch coating 6 by-the coating apparatus 15 which is moved fromone end of the top surface to the other in the course of the coatingoperation.

The burner manifold 16 provides flames 18 of gradually increasinglengths to superficially preheat the potlining surface as it moves alongit. The asbestos sheet 19 heat insulates the burner from the metalframework 21, which houses a freely moving block of graphitepitchmixture 22, which has been proportioned, premixed and precast in a shapeof slightly less horizontal cross sectiondimension than the framework21. Mixture 22 is pushed downward against the hot potlining surface by aspring 23, so that the block of graphitepitch mixture melts and spreadsthe coating 6 uniformly over the top of the potlining sealing itssurface against penetration. It will be appreciated that when a liquidsuspension is applied, as with aluminum fluoridepitch, framework 21 willbe an open-bottomed container and spring 23 would be replaced withstirring means. After the coating 6 is applied, a second burner 25 burnsout the volatiles therein.

Optionally, the absestos board 19 may have a cupshaped surface 24 whichrubs on the anode surface as it moves with the apparatus 15. The cuppedsurface is evacuated through pipesand pumps (now shown) so that thegraphite-pitch mixture which melts is forced into the surfaceinterstices by atmospheric pressure. The creation of a more perfectvacuum beneath the plastic mixture is aided by the subsequent cooling ofthe potlining surface in which the remaining gases in the potliningsurface contract and the volatiles conarrangements of parts, which havebeen herein described and illustrated in order to explain the nature ofthe invention, may be made by those skilled in the art within theprinciple and scope of the invention as defined in the appended claims.

For example, unbaked anodes or cathodes (as well as baked anodes andcathodes) may be impregnated or coated with the materials and in amanner similar to that described above.

What is claimed is:

1. In the operation of an aluminum-reduction cell for the electrolyticreduction of aluminum from a molten cryolite bath containing dissolvedalumina, current therefore passing from carbonaceous anodes through thesaid bath to a carbonaceous cathodic lining, and wherein ingredients ofsaid bath are gradually volatilized and absorbed into said lining, theimprovement comprising continually providing make-up ingredients forsaid bath by utilizing anodes having said make-up ingredients in theform of particulate material impregnated into the sides thereof with apitch binder, said impregnated material also acting to reduce oxidationof said anodes during operation of said cell and erosion of said anodesin contact with said bath.

2. The method as claimed in claim 1, wherein said pitch is ahigh-melting, low-volatile coal tar pitch.

3. The method as claimed in claim 1, wherein said make-up ingredientsare selected from the group consisting of cryolite, aluminum fluoride,sodium fluoride, sodium carbonate, calcium fluoride, lithium fluoride,magnesium fluoride and alumina.

4. The method as claimed in claim 3, wherein said impregnated materialcomprises a first, penetrating layer of aluminum fluoride and a second,covering layer of one or more of said make-up ingredients.

2. The method as claimed in claim 1, wherein said pitch is ahigh-melting, low-volatile coal tar pitch.
 3. The method as claimed inclaim 1, wherein said make-up ingredients are selected from the groupconsisting of cryolite, aluminum fluoride, sodium fluoride, sodiumcarbonate, calcium fluoride, lithium fluoride, magnesium fluoride andalumina.
 4. The method as claimed in claim 3, wherein said impregnatedmaterial comprises a first, penetrating layer of aluminum fluoride and asecond, covering layer of one or more of said make-up ingredients.